Prior techniques have been developed for displaying information in the field of view of binoculars, night vision goggles, weapon sights, and similar hand-held or helmet mounted optical viewing devices. One prior technique utilized a fixed reticle imposed in the optical path which would appear to the eye of the viewer. Such a fixed reticle could be permanently etched on a lens surface, for example. Alternatively, the reticle might comprise a wire or small symbol implanted in the optical path. A disadvantage with such a fixed reticle is that the etched or implanted indicia is always visible to the viewer and cannot be readily removed when it is not desired to use the indicia. In addition, the use of such fixed reticle has heretofore obscured the field of view and the actual scene being viewed. Finally, the displayed indicia could not be readily changed or varied by the user.
Another prior art technique is the use of a light emitting diode (LED) which is implanted within the optical system. When energized, the LED causes a small, illuminated dot to be displayed to the viewer. The illuminated dot might indicate, for example, the energization of a particular feature of the optical viewing device. The LED display, unlike the fixed reticle discussed above, could be de-energized and removed from the field of view when not needed or wanted. However, the LED display still suffers the disadvantage of being unable to display changing or variable information.
Other prior techniques have included "heads up" displays for pilots of aircraft wherein information is displayed adjacent to the windshield of the aircraft, or, in some instances, through a viewer worn by the pilot. However, these techniques have not been used in conjunction with self contained optical viewing devices because the apparatus required is quite large and cannot be reduced in size sufficiently to be packaged in a hand-held or helmet mounted optical viewing device. Similarly, the automotive industry uses a projection device to display the speed of an automobile on the windshield. However, this device is also much too large and impractical for implementation in an optical viewing device such as a night vision goggle or binocular viewer.
One particularly useful type of information which would be desirable to display in the field of view of an optical viewing device, such as a night vision goggle or binocular, is a compass heading. Compass heading information viewable through a night vision goggle would be especially useful, because at night a goggle user would be unable to obtain his geographical orientation from the location of the sun, and because it is impractical and inconvenient to remove a helmet-mounted night vision goggle to refer to a handheld, magnetic compass.
Electronic compasses which generate digital representations of compass headings are in the prior art. Some of the uses of these compasses have been within automobiles. An example of such an electronic compass is disclosed in U.S. Pat. No. 5,046,260 issued to Wellhausen on Sep. 10, 1991. However, previous electronic compasses have been relatively bulky and too large to incorporate in small optical viewing devices. In addition, there have been no previously known practical techniques for selectively projecting the output from an electronic compass into the optical path of an optical viewing device so that the user could selectively view the field of view while simultaneously viewing compass heading.
Another useful type of information for displaying in the field of view of an optical viewing device is stadiametric ranging information. This information is particularly useful to a foot soldier using a binocular or a night vision goggle in a combat situation. The soldier could use measuring indicia within the field of view of his goggle to quickly determine his approximate distance from an enemy soldier or tank, for example.
Stadiametric ranging has been heretofore provided to the user of a weapon sight through the use of a fixed reticle projected through the image tube and displayed to the viewer of the weapon sight. However, fixed reticle ranging displays are undesirable because they cannot be removed from the field of view when not needed, and therefore would tend to obstruct the vision of the user when it is not necessary to have the range information displayed. In addition, it is difficult to provide a fixed reticle in a two tube night vision goggle.
Consequently, a need exists for an improved optical viewing device in which variable information, such as compass heading and/or stadiametric range determining information, can be selectively optically overlaid or superimposed upon the image of a scene viewed through the device. Preferably, the superimposed information would be selectively presented or removed by the user so as not to interfere with the user's vision when the information is not needed. Also, in devices such as night vision goggles in which the image of a scene is typically presented to the user in a monochrome color, the superimposed information will preferably be displayed in a contrasting color for improved readability